Automatic transmissions of motor vehicles, especially of commercial vehicles, generally have assemblies consisting of subassemblies, which can be handled individually, for actuating at least one double-acting shift cylinder of a shift assembly. Such assemblies are known, for example, from DE 10 2005 015 481 A1 and DE 199 50 443 A1. The assemblies have at least two control valves by which a double-acting shift cylinder can be subjected to pneumatic or hydraulic pressure. A piston rod of a piston of the shift cylinder is generally connected to a shift fork, a shift finger or similar actuating member by means of which a transmission path can be selected as required. Alternating activation of the control valves ensures that one pressure chamber of the shift cylinder is subjected to hydraulic pressure while the other pressure chamber of the shift cylinder is vented; that is, pneumatic or hydraulic, pressure is discharged from that pressure chamber, the hydraulic fluid, in the case of hydraulic systems, being returned to a hydraulic fluid reservoir via a return line.
The shift cylinders are generally configured as double-acting, cylinders in the form of differential cylinders, and accordingly have a piston with two effective areas of different sizes, since only one effective area of the piston is connected to a piston rod.
In the case of hydraulic assemblies, a pressure-limiting valve, which ensures that the pressure supply line is connected to the return line in the event of an overpressure, is generally provided, so that a possibly harmful overpressure is removed from the system. Otherwise, bursting of hydraulic lines or damage to the control valves or the shift cylinder might occur.
Such dangerous overpressure situations can occur if the motor of the hydraulic pump is energized continuously as a result of a mechanical or electronic fault, so that hydraulic fluid is then pumped constantly into the pressure supply line.
It is therefore usual to incorporate a pressure-limiting valve in a corresponding assembly, as is known, for example, from Applicant's DE 199 31 973 A1 and from DE 10 2006 031 380 A1.
An assembly in which a pressure-limiting valve is not provided is known from DE 10 2006 058 913 A1, according to FIG. 1 of that document. Such an assembly offers no protection against hydraulic overpressure.
These conventional assemblies in which pressure-limiting valves are used have proved widely effective in practice. However, the overpressure valve in such constructions is typically designed such that it is shut down in normal operation and is switched only in an extreme exceptional case, so that it can perform its function in an emergency. Such extreme cases in which the system is exposed to an unnaturally high hydraulic pressure are fortunately extremely rare. However, since the corresponding overpressure valves are normally not actuated over several years, it may happen that, when the extreme case occurs, the pressure-limiting valve is no longer operative, which may result from valve elements being no longer movable through corrosive or other influences. This may accordingly lead to unacceptable damage to the system, manifested, for example, in hydraulic lines bursting or the shift cylinder being actuated in the presence of especially high overpressure, whereby a gear is selected or unselected in an unwanted manner, with resulting severe damage to the transmission and the vehicle.